This is not true. Pressure in the tank does not tell you how much gas is left, and pressure gauges for this purpose cannot be bought (for propane). Pressure in the tank is dependent on temperature, but will be constant at a given temperature, as long as there is any liquid propane in the tank. To determine how much propane is left, you need to weigh the tank. This needs an example for which the gas does not liquefy under the relevant pressures--maybe a compressor for air tools?